1. Field of the Invention
The present invention relates to a sealed terminal having the improved anticorrosive characteristic and water sealing characteristic.
2. Description of the Prior Art
A sealed terminal has been widely used for various electric apparatuses which require air tightness or water sealing characteristics. The present invention will be illustrated in the case of an auxiliary anode which is used for electric hot water apparatuses.
Apparatuses for containing water include electric hot water apparatuses using electric power at night and hot water apparatuses using heat source of petroleum and gas. These hot water apparatuses have been widely used. A metal vessel is usually equipped in these hot water apparatuses and enamel coated vessels or stainless steel vessels have been used because of anticorrosive characteristic of the vessels.
These vessels usually have a closed structure having no opening and accordingly, they have a welded part. Even though stainless steel is used, the corrosion of the welded part is disadvantageously found. When an enamel coated plate is used, the uncoated part is always found at the welded part whereby the corrosion can not be prevented.
In general, metal dipped in water has a specific potential because of ionization of the metal. The potential is referred to as galvanic potential or corrosion potential, and the corrosion is caused by the corrosion potential. In order to prevent the corrosion, the current is fed from the other electrode for inhibiting the ionization of the metal. A galvanic anode system and an impressed current system have been known as the cathodic protection method in the principle.
In the former system, a metal which has lower corrosion potential and is easily ionized is electrically contacted to form an electric cell so as to provide a resulting suitable current. In the conventional hot water apparatus, the former system has been widely employed. A magnesium metal rod has been used as the galvanic anode. The galvanic anode requires a large surface area so as to generate current needed for the protection. When the vessel is large, it is necessary to use a magnesium metal rod having a larger diameter or to increase the number of the magnesium metal rods and, accordingly, the same is expensive. Sometimes, the magnesium metal rod is damaged by partial consumption whereby the protective effect is remarkably small or is disadvantageously not to be found. A further disadvantage is the dissolving of magnesium in water whereby the quality of water is inferior.
In the impressed current system, an electrode is inserted into the vessel containing water in simple structure whereby suitable power voltage is applied from the outside to form the anode and the current required for protection is obtained. When the vessel is larger, it is enough to control the potential so as to control the current and the number of the electrodes need not increase since the related cost is not as high as the galvanic anode system.
The latter system is excellent in overcoming the disadvantages of the life of magnesium metal rods such as the disappearance of the function caused by falling down or the change of quality of water caused by dissolving magnesium.
Although the impressed current system has a remarkable effect in principle, the system has not been practically applied because a sealed terminal or the electrode equipped in the sealed terminal has not yet been obtained.
The main characteristic required for the electrode is to have smaller consumption as the anode. The main characteristic required for the sealed terminal is to have enough insulation with respect to the vessel and to maintain the air-tightness or the water sealing characteristic as well as bearing high thermal shock. It is preferable to prevent the deterioration of characteristics of the sealed terminal and the electrode in aging. It is especially effective to use the electrode rod which is used as the terminal conductor of the sealed terminal in one piece structure so as to decrease the processing steps and to reduce the cost.
In order to satisfy the characteristics, the electrode rod used as the anode is preferably made of platinum which maintains perfect anticorrosive characteristic and the non-consumption characteristic, however, it is disadvantageously expensive. On the other hand, a metal coated with platinum such as a titanium wire coated with platinum has excellent anticorrosive characteristics and can be used under high current density and can be advantageously used as the electrode rod from the viewpoints of these characteristics as well as the cost.
On the other hand, the characteristics of the insulator for seal-bonding the terminal conductor and the sealing structure are important for the sealed terminal.
Referring to the drawings, the conventional embodiments will be discussed. FIGS. 1 to 4 are respectively the sectional views of the conventional embodiments of the sealed terminal used for the hot water apparatus.
In FIG. 1, the reference numeral (1) designates a terminal conductor which is also used as the electrode rod; (2) designates a substrate made of iron which has flange; (3) designates a terminal opening forward in the substrate; (4) designates an insulator which is inserted into the terminal opening and is made of porcelain for holding the terminal conductor (1) under insulation by the hole formed at the center; (5) and (5') respectively designate metallic film formed on the surface of the insulator; (6) designates a base metal which is welded with braze to the terminal conductor; (7) and (7') respectively braze for sealing the substrate (2), the insulator (4) and the terminal conductor by the welding; and (8) designates a screw formed outer surface of the substrate (2). In the conventional embodiment, the terminal conductor (1) is also used as the electrode rod. The reference (9) designates a metal vessel.
In the conventional sealed terminal, the metallic film (5), (5') or the base metal (6) is used for brazing the terminal conductor (1), the insulator (4) and the substrate (2). However, the disadvantage of limitation of the material is found because of the indispensable condition based on the difference of coefficients of linear expansion of the parts. For example, even though the titanium wire coated by the platinum plating which has an excellent anticorrosive characteristic is preferable as the terminal conductor (1), it could not be used in practice because the platinum coat is peeled off in the welding with braze. The base metal for welding and the braze (7') are disposed in the anode side to expose it to hot water, whereby the anticorrosive characteristic as the electrode is inferior. The anticorrosive characteristic is required only in the side of the terminal conductor (1) as the anode without the relation of the substrate (2) which is electrically insulated. The mutual connection of the substrate (2), the insulator (4) and the terminal conductor (1) is provided by the braze (7') through the metallic film (5) and the basic metal (6) for welding whereby the mechanical strength is disadvantageously quite low.
In the conventional embodiment shown in FIG. 2, the parts (1) and (4) to (9) are identical or corresponding to the parts in FIG. 1. The insulator (4) made of porcelain is held in sealing with a pair of packings (10), (10') and the screw (11) for fitting the packings. In the conventional embodiment of the sealed terminal having the above-mentioned structure, the disadvantages of the limitation of the material for welding with braze and the strength and the low anticorrosive characteristic caused by contacting the welded part with the hot water are also found to be the same as those of FIG. 1. Moreover, the number of the parts is increased so as to complicate the assembly.
In the conventional embodiment of the sealed terminal shown in FIG. 3, the parts (1), (2), (4) and (9) to (11) are identical or closely corresponding to the parts in FIG. 2. The reference (12) designates an electrode rod which is welded on the terminal conductor (1). In this case, Fernico is used as the terminal conductor (1) and the insulator (4) is made of glassy material which seals the terminal conductor (1) with the substrate (2). The substrate (2) is held on the body of the vessel through the packings (10) and (10') by the screw (11).
In the conventional embodiment, the material for the terminal conductor (1) is limited to Fernico etc. from the viewpoint of the coefficient of linear expansion. Even though the titanium wire coated by the platinum plating which has excellent anticorrosive characteristic is used as the electrode rod (12), the terminal conductor made of Fernico etc. is exposed into the hot water whereby the anticorrosive characteristic is inferior and many parts such as packings (10) are used disadvantageously.
In the conventional embodiment shown in FIG. 4, the parts (1) to (4), (8), (9) and (12) are identical or corresponding to the parts in FIG. 3. The references (13), and (13') respectively glassy sealing composition for seal-bonding the insulator (4) made of porcelain and the electrode rod (12) made of conductive ferrite, and for seal-bonding the insulator (4) and the substrate (2). The reference (14) designates a filler made of cured resin for fixing the connecting part between electrode rod (12) and the terminal conductor (1), at the terminal opening (3).
In the conventional embodiment, the electrode rod (12) made of the conductive ferrite has the anticorrosive characteristic being inferior to the titanium wire coated by the platinum plating but superior to the Fernico or the braze. However, the electrode rod (12) has inferior mechanical strength and thermal shock resistance whereby it it easily broken by slight shock at the assemble. When this is equipped in the hot water apparatus, this is easily broken near the contact to the insulator (4) by repeating the heating and cooling cycles. The electric resistance of the electrode rod is remarkably high in comparison with the other metals whereby disadvantageously the current is limited.
The necessity for improving insulation between the terminal conductor and the metal vessel will be explained as follows.
FIG. 5 is a schematic view for describing the principle of the outer power source system. In FIG. 5, the reference (20) designates a DC power source while (21) designates the welded part.
Even though the inner wall of the metal vessel (9) is coated with enamel, the metal is exposed at the welded part (21) so as to cause corrosion. In order to prevent the corrosion of the welded part (21), the terminal conductor (1) is disposed in the connection to the DC power source (20) to give positive potential to the metal vessel (9).
The characteristics required for the terminal conductor (1) are basic characteristics such as less consumption of the electrode, high thermal shock resistance, high mechanical strength and complete water sealing characteristic as well as easy assemble in the wall when it is used in practice. The reliability for extended time is quite important.
The most important problem with respect to reliability is the insulating characteristic between the anode and the cathode.
In such anticorrosive device, the terminal conductor (1) as the anode is disposed at the center and the substrate (2), being short-circuited to the cathode is disposed at the peripheral part and the insulator (4) is filled in the space. The substrate (2) is directly connected on the wall of the metal vessel (9).
The insulation resistance is mainly dependent upon the creeping resistance of the water-contacting surface of the insulator (4) disposed between the electrodes. From the viewpoint of reliability for long time, it is not true that the creeping resistance is considered to be the most important factor.
The reason for this will be described hereinafter. When the anticorrosive electrode is used for the vessel in the condition of complete control of water as a large boiler, there is less problem. However, when it is used for a house-hold hot water apparatus using electric power at night, the problem is serious. Various kinds of water is supplied to the hot water apparatus and water having high permanent hardness or temporary hardness is used, or water containing various ionized materials, whereby the material formed by the electrolysis is deposited around the electrodes. The deposited material has high electric conductivity whereby the insulation resistance between the electrodes is lowered. The phenomenon is dependent upon the time of use and is disadvantageous from the viewpoint of long reliability.
In the electrode structure shown in FIG. 5, the materials formed by the electrolysis are deposited on the peripheral parts (22) between the electrodes whereby the insulation resistance between the electrodes are decreased and the long period reliability is unsatisfactory.